EP3395987A1 - Hochfeste stahlplatte mit niedriger streckgrenze und ausgezeichneter spannungsrisskorrosionsbeständigkeit und niedriger temperaturzähigkeit - Google Patents
Hochfeste stahlplatte mit niedriger streckgrenze und ausgezeichneter spannungsrisskorrosionsbeständigkeit und niedriger temperaturzähigkeit Download PDFInfo
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- EP3395987A1 EP3395987A1 EP16879393.3A EP16879393A EP3395987A1 EP 3395987 A1 EP3395987 A1 EP 3395987A1 EP 16879393 A EP16879393 A EP 16879393A EP 3395987 A1 EP3395987 A1 EP 3395987A1
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- steel
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 81
- 239000010959 steel Substances 0.000 title claims abstract description 81
- 230000007797 corrosion Effects 0.000 title claims abstract description 43
- 238000005260 corrosion Methods 0.000 title claims abstract description 43
- 238000005336 cracking Methods 0.000 title claims abstract description 40
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims abstract description 32
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 21
- 239000011572 manganese Substances 0.000 claims abstract description 19
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 19
- 239000010955 niobium Substances 0.000 claims abstract description 17
- 239000010936 titanium Substances 0.000 claims abstract description 17
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 15
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims abstract description 12
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 12
- 239000011574 phosphorus Substances 0.000 claims abstract description 12
- 229910001568 polygonal ferrite Inorganic materials 0.000 claims abstract description 11
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims abstract description 10
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims abstract description 10
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 10
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims abstract description 9
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims abstract description 9
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 9
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 9
- 239000011593 sulfur Substances 0.000 claims abstract description 9
- 229910052719 titanium Inorganic materials 0.000 claims abstract description 9
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims abstract description 8
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 8
- 239000012535 impurity Substances 0.000 claims abstract description 8
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 8
- 239000010703 silicon Substances 0.000 claims abstract description 8
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 7
- 239000000470 constituent Substances 0.000 claims abstract description 7
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 7
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 7
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims abstract description 7
- 238000005096 rolling process Methods 0.000 claims description 56
- 238000001816 cooling Methods 0.000 claims description 42
- 238000004519 manufacturing process Methods 0.000 claims description 20
- 238000000034 method Methods 0.000 claims description 16
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- 230000001186 cumulative effect Effects 0.000 claims description 5
- 230000000052 comparative effect Effects 0.000 description 49
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 44
- 229910021529 ammonia Inorganic materials 0.000 description 21
- 230000000694 effects Effects 0.000 description 12
- 239000000203 mixture Substances 0.000 description 12
- 230000001965 increasing effect Effects 0.000 description 9
- 239000007789 gas Substances 0.000 description 8
- 239000000463 material Substances 0.000 description 6
- 229910045601 alloy Inorganic materials 0.000 description 5
- 239000000956 alloy Substances 0.000 description 5
- 229910000734 martensite Inorganic materials 0.000 description 5
- 238000005728 strengthening Methods 0.000 description 5
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 4
- 239000000243 solution Substances 0.000 description 4
- 229910001562 pearlite Inorganic materials 0.000 description 3
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- 238000009628 steelmaking Methods 0.000 description 2
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- 239000012085 test solution Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000003466 welding Methods 0.000 description 2
- BWSQKOKULIALEW-UHFFFAOYSA-N 2-[2-[4-fluoro-3-(trifluoromethyl)phenyl]-3-[2-(piperidin-3-ylamino)pyrimidin-4-yl]imidazol-4-yl]acetonitrile Chemical compound FC1=C(C=C(C=C1)C=1N(C(=CN=1)CC#N)C1=NC(=NC=C1)NC1CNCCC1)C(F)(F)F BWSQKOKULIALEW-UHFFFAOYSA-N 0.000 description 1
- AETVBWZVKDOWHH-UHFFFAOYSA-N 2-[4-[2-(2,3-dihydro-1H-inden-2-ylamino)pyrimidin-5-yl]-3-(1-ethylazetidin-3-yl)oxypyrazol-1-yl]-1-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)ethanone Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)C=1C(=NN(C=1)CC(=O)N1CC2=C(CC1)NN=N2)OC1CN(C1)CC AETVBWZVKDOWHH-UHFFFAOYSA-N 0.000 description 1
- ZYPDJSJJXZWZJJ-UHFFFAOYSA-N 2-[4-[2-(2,3-dihydro-1H-inden-2-ylamino)pyrimidin-5-yl]-3-piperidin-4-yloxypyrazol-1-yl]-1-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)ethanone Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)C=1C(=NN(C=1)CC(=O)N1CC2=C(CC1)NN=N2)OC1CCNCC1 ZYPDJSJJXZWZJJ-UHFFFAOYSA-N 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- BVCZEBOGSOYJJT-UHFFFAOYSA-N ammonium carbamate Chemical compound [NH4+].NC([O-])=O BVCZEBOGSOYJJT-UHFFFAOYSA-N 0.000 description 1
- -1 and to this end Inorganic materials 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- KXDHJXZQYSOELW-UHFFFAOYSA-N carbonic acid monoamide Natural products NC(O)=O KXDHJXZQYSOELW-UHFFFAOYSA-N 0.000 description 1
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- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 1
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- 238000009864 tensile test Methods 0.000 description 1
Images
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present disclosure relates to a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness.
- a temperature for liquefying a gas is generally low (-52°C in the case of LPG) at normal pressure, and thus, steel used in a liquefied gas storage tank has been required to have excellent low temperature toughness in a welded part, as well as in a base material.
- IGC CODE International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk
- methods for removing stress from a welded part include a post welding heat treatment (PWHT) based on a heat treatment and a mechanical stress relief (MSR) method of removing stress by adding hydrostatic pressure, or the like, to the welded part, or the like.
- PWHT post welding heat treatment
- MSR mechanical stress relief
- yield strength and tensile stress are limited, to be significantly different.
- gas tanks are basically required to be enlarged in size, it may be difficult to remove stress by the PWHT method and most shipbuilders prefer the MSR method, and thus, steel for manufacturing gas tanks is required to have low yield ratio characteristics.
- Patent document 1 proposes a technique of adding 6.5 to 12.0% of Ni to achieve excellent low temperature toughness.
- Patent document 2 proposes a technique of mixedly using tempered martensite and bainite by performing quench tempering on steel having a specific composition.
- Patent document 1 has a problem of low economical efficiency due to high-priced Ni content and has a problem of degrading stress corrosion cracking (SCC) resistance.
- Patent document 3 proposes a technique of only softening a surface layer of a steel sheet to realize a low-yield ratio. This technique, however, may achieve low temperature toughness and low yield ratio separately but cannot obtain both low temperature toughness and low yield ratio together.
- precipitation strengthening, solid solution strengthening, and martensite strengthening may be used but these methods degrade toughness and elongation, while enhancing strength.
- An aspect of the present disclosure is to provide a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, and a manufacturing method thereof.
- a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness includes: by weight percent (or percent by weight) (wt%), 0.02 to 0.10% of carbon (C), 0.5 to 2.0% of manganese (Mn), 0.05 to 0.5% of silicon (Si), 0.05 to 1.0% of nickel (Ni), 0.005 to 0.1% of titanium (Ti), 0.005 to 0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), a balance of Fe and other inevitable impurities, and a microstructure includes, in area percent (%), 60% or more of acicular ferrite and the balance including at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA).
- a method of manufacturing a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness includes: heating a slab including, by weight percent (or percent by weight) (wt%), 0.02 to 0.10% of carbon (C), 0.5 to 2.0% of manganese (Mn), 0.05 to 0.5% of silicon (Si), 0.05 to 1.0% of nickel (Ni), 0.005 to 0.1% of titanium (Ti), 0.005 to 0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), a balance of Fe and other inevitable impurities, to 1000 to 1200°C; rough-rolling the heated slab at a temperature of 1100 to 900°C; finishing-rolling at a temperature between Ar3 + 100°C and Ar3 + 30°C on the basis of a center temperature after the rough rolling; and cooling to a temperature of 300°C or lower
- the low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness and the manufacturing method thereof may be provided.
- the inventors of the present application recognized that it is difficult to make both ammonia stress corrosion cracking resistance and low temperature toughness excellent and have studied to solve the problem.
- the inventors confirmed that it is possible to provide a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness by controlling an alloy composition and a microstructure and a manufacturing method thereof, thereby completing the present disclosure.
- the low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness includes, by weight percent (or percent by weight) (wt%), 0.02 to 0.10% of carbon (C), 0.5 to 2.0% of manganese (Mn), 0.05 to 0.5% of silicon (Si), 0.05 to 1.0% of nickel (Ni), 0.005 to 0.1% of titanium (Ti), 0.005 to 0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), a balance of Fe and other inevitable impurities.
- a microstructure includes, in area percent (%), 60% or more of acicular ferrite and the balance including at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA).
- C is the most important element for securing basic strength, it is necessary to be contained within an appropriate range in the steel, and in order to obtain an additive effect, preferably, C is added in an amount of 0.02% or more.
- the C content is less than 0.02%, strength may be reduced and the yield ratio may be lowered, which is not preferable. If the C content exceeds 0.10%, a large amount of low temperature transformation phases such as bainite, or the like, is generated to exceed an upper limit of yield strength that may cause ammonia stress corrosion cracking (SCC).
- SCC stress corrosion cracking
- the content of C is preferably limited to 0.02 to 0.10%. More preferably, it is 0.05 to 0.08%.
- Si has an effect of increasing strength due to the effect of solid solution strengthening and is advantageously used as a deoxidizing agent in steel making process.
- the Si content is less than 0.05%, the deoxidation effect and the strength improving effect may be insufficient. If the Si content exceeds 0.5%, the low-temperature toughness is lowered and weldability is deteriorated.
- the silicon content is preferably limited to 0.05 to 0.5%. More preferably, it is 0.05 to 0.3%.
- Manganese contributes to ferrite grain refinement and is an element useful for improving strength by solid solution strengthening.
- manganese In order to obtain the effect of manganese, manganese is required to be added in an amount of 0.5% or more. If, however, the content exceeds 2.0%, hardenability may be excessively increased, which promotes formation of upper bainite and martensite to significantly reduce impact toughness and ammonia stress corrosion cracking (SCC) resistance and to reduce toughness of weld heat-affected zone as well.
- SCC stress corrosion cracking
- the Mn content is preferably limited to 0.5 to 2.0%. More preferably, it is 1.0 to 1.5%.
- Ni is an important element for facilitating cross slip of dislocations at low temperatures to improve impact toughness and hardenability and to improve strength. In order to obtain such an effect, Ni is preferably added in an amount of 0.05% or more. If the Ni content exceeds 1.0%, ammonia stress corrosion cracking (SCC) may occur and manufacturing costs may be increased due to the high cost of Ni relative to other hardenable elements.
- SCC stress corrosion cracking
- the Ni content is preferably limited to 0.05 to 1.0%, and more preferably, 0.2 to 0.5%.
- Nb dissolved in reheating at high temperatures is precipitated very finely in the form of NbC to inhibit the recrystallization of austenite, thereby making the structure finer.
- Nb is preferably controlled to 0.005% or less. More preferably, it is 0.003% or less.
- Titanium forms oxides and nitrides in the steel to inhibit growth of crystal grains during reheating, thereby significantly improving low temperature toughness, and is also effective in refining the microstructure of a welded portion.
- titanium In order to obtain such an effect, titanium needs to be added in an amount of 0.005 wt% or more. If the content exceeds 0.1 wt%, low temperature toughness may be reduced due to clogging of a nozzle or crystallization of a central portion.
- the titanium content is preferably 0.005 to 0.1%. More preferably, it is 0.01 to 0.03%.
- Aluminum is an element useful for deoxidizing molten steel, and to this end, aluminum needs to be added in an amount of 0.005 wt% or more. If the content exceeds 0.5 wt%, nozzle clogging may occur during continuous casting. Therefore, the aluminum content is preferably 0.005 to 0.5%. More preferably, it is 0.005 to 0.05%.
- Phosphorus is an element that causes grain boundary segregation in a base material and a welded portion. Since phosphorus causes a problem of embrittling steel, an amount of phosphorus needs to be actively reduced. However, reducing phosphorus to an extreme limit may deepen a load of a steel making process and since the aforementioned problem does not significantly arise as long as the content of phosphorus is 0.015% or less, an upper limit thereof is limited to 0.015%, more preferably, to 0.010%.
- S Sulfur
- MnS metal-oxide-semiconductor
- sulfur is preferably controlled to as low as possible and the content is limited to 0.015 wt% or less, more preferably, to 0.005 wt%.
- the balance of the present disclosure is iron (Fe) .
- impurities may be inevitably incorporated from a raw material or a surrounding environment, which may not be excluded. These impurities are known to any one skilled in the art in the ordinary manufacturing process and thus not specifically mentioned in this disclosure.
- the microstructure of the steel of the present disclosure includes, in area %, 60% or more of acicular ferrite and a balance of at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA).
- the area fraction of the acicular ferrite is preferably 60% or more.
- the inclusion of pearlite may lower tensile strength and low-temperature impact toughness, and thus, the microstructure of the steel of the present disclosure may not contain pearlite.
- the acicular ferrite measured in terms of the equivalent of a circle diameter may be 30 ⁇ m or less. If the size exceeds 30pm, impact toughness may be lowered.
- bainite is preferably granular bainite and upper bainite.
- an area fraction of the bainite is preferably 30% or less. If the area fraction of the bainite exceeds 30%, an upper limit (440 MPa) of yield strength (440 MPa) which may cause ammonia stress corrosion cracking (SCC) may be exceeded, and thus, it is necessary to limit the fraction of the bainite.
- an upper limit (440 MPa) of yield strength (440 MPa) which may cause ammonia stress corrosion cracking (SCC) may be exceeded, and thus, it is necessary to limit the fraction of the bainite.
- the MA phase is preferably 10% by area or less and the size measured by the equivalent of a circle diameter is preferably 5 ⁇ m or less.
- MA Martensite-Austenite constituent
- MA martensitic island.
- the steel of the present disclosure satisfying the above conditions may have a yield ratio (YS/TS) of 0.85 or less, preferably, 0.8 or less.
- the steel may have tensile strength of 490 MPa or greater, for example, about 510 to 610 MPa, having excellent tensile strength.
- an upper limit of yield strength of the steel is 440 MPa or less and does not exceed the upper limit of yield strength which causes ammonia stress corrosion cracking (SCC), and thus, ammonia stress corrosion cracking (SCC) resistance may be excellent.
- an impact transition temperature of the 1/4t portion in a thickness direction of the steel is -60°C or lower, low temperature toughness may be excellent.
- t represents a thickness of the steel.
- the steel has a thickness of 6 mm or greater, and preferably, 6 to 50 mm.
- the steel of the present disclosure may secure all of high strength, low yield ratio, excellent low temperature toughness, and ammonia stress corrosion cracking (SCC) resistance.
- the method of manufacturing a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness includes : heating a slab having the above-described alloy composition to 1000 to 1200°C; rough-rolling the heated slab at a temperature of 1100 to 900°C; finishing-rolling at a temperature between Ar3 + 100°C and Ar3 + 30°C on the basis of a center temperature after the rough rolling; and cooling to a temperature of 300°C or lower after the finishing-rolling.
- the slab having the above-described alloy composition is heated to 1000 to 1200°C.
- the heating temperature of the slab is preferably 1000°C or higher, and this is to dissolve a Ti carbonitride formed during casting. If the heating temperature of the slab is too low, deformation resistance during rolling is too high, so that a reduction ratio per rolling pass may not be increased in a follow-up rolling process, and thus, a lower limit thereof is preferably limited to 1000°C. However, if heating is carried out at an excessively high temperature, austenite may be coarsened to lower toughness, and thus, an upper limit of the heating temperature is preferably 1200°C.
- the heated slab is subjected to rough rolling at a temperature of 1100 to 900°C.
- the rough rolling temperature is preferably set to be not lower than a temperature (Tnr) at which recrystallization of the austenite is stopped.
- Tnr a temperature at which recrystallization of the austenite is stopped.
- An effect of breaking a cast structure such as dendrites formed during casting and reducing the size of austenite may be obtained through rolling.
- the rough rolling temperature is preferably limited to 1100 to 900°C.
- the rough rolling may be performed so that the last three rolling passes have a reduction ratio of 10% or greater per pass.
- the reduction ratio per pass is at least 10% and a total cumulative reduction ratio is at least 30% for the last three rolling passes during rough rolling.
- the reduction ratio per pass in rough rolling is lowered, sufficient deformation is not transferred to the central portion, which may cause toughness degradation due to center coarsening. Therefore, the reduction ratio per pass of the last three passes is preferably limited to 10% or greater.
- a cumulative rolling reduction ratio at the time of rough rolling it is preferable to set a cumulative rolling reduction ratio at the time of rough rolling to 30% or greater.
- finishing rolling is performed at a temperature between Ar3 + 100°C and Ar3 + 30°C on the basis of a temperature of the central portion.
- finishing rolling If the temperature for finishing rolling is lowered to below Ar3 + 30°C, the ferrite grain size becomes too fine to exceed the yield strength upper limit (440 MPa) causing ammonia stress corrosion cracking (SCC). Also, finishing rolling performed at a temperature exceeding Ar3 + 100°C is not effective in miniaturizing the grain size. Thus, it is preferable to carry out the finishing rolling at a temperature between Ar3 +100°C and Ar3 +30°C and a microstructure of the steel sheet to be subjected to finishing rolling under such conditions may be a composite structure having the features mentioned above.
- the cumulative reduction ratio at 60% or greater during finishing rolling and to maintain the reduction ratio per pass, excluding the final shape sizing phase, at 10% or more .
- the steel sheet After the finishing rolling, the steel sheet is cooled to a temperature of 300°C or lower.
- the cooling is preferably started at a temperature of Ar3+30°C to Ar3 and cooled to a finish cooling temperature (FCT) of 300°C or lower, for example, about 100 to 300°C.
- FCT finish cooling temperature
- the finish cooling temperature is higher than 300°C, the fine MA phase may be decomposed due to a tempering effect to make it difficult to realize a low yield ratio.
- the finish cooling temperature is preferably 300°C or lower.
- first cooling may be performed such that a cooling rate at the central portion is 15°C/s or greater up to Bs-10°C to Bs+10
- second cooling may be performed up to 300°C or lower such that a cooling rate at the central portion is 10 to 50°C/s.
- the cooling start temperature may be Ar3 + 30°C to Ar3.
- the above-mentioned first cooling preferably starts, after finishing rolling, to perform cooling at a temperature of Ar3 + 30°C to Ar3 up to Bs-10°C at a cooling rate of 15°C/s or higher, for example, 30°C/s or higher, in the central portion of the steel sheet.
- the cooling rate of the central portion of the steel sheet is lower than 15°C/s up to Bs-10°C to Bs+10°C in the first cooling, it is possible to form a coarse polygonal ferrite to lower tensile strength and impact toughness.
- the second cooling is preferably performed after the first cooling up to the finish cooling temperature of 300°C or lower, for example, 100 to 300°C, at a cooling rate of 10°C/s to 50°C/s in the central portion of the steel sheet.
- the bainite fraction is formed to be 30% or greater by area as in the microstructure of 1- (1) of FIG. 1 to exceed the yield strength upper limit (440 MPa) causing ammonia stress corrosion cracking (SCC), and the excessive increase in strength may lower elongation and impact toughness .
- a coarse polygonal ferrite and pearlite rather than the fine acicular ferrite like the microstructure of 1-(3) of FIG. 1 , may be formed, leading to a possibility that tensile strength is 490 MPa or less and Charpy transition temperature is -60°C or higher.
- a 300 mm-thick steel slab having the composition shown in Table 1 below was reheated to a temperature of 1100°C and then subjected to rough rolling at a temperature of 1050°C to prepare a bar. A cumulative reduction ratio during rough rolling was applied equally as 30%. Also, Ar3 and Bs temperatures according to compositions of each steel were calculated and are shown in Table 1 below.
- finishing rolling was performed to satisfy the difference between the finishing rolling temperature and the Ar3 temperature shown in Table 2 below to obtain a steel sheet having the thickness shown in Table 2, and thereafter, cooling performed at various cooling rates through multistage cooling.
- a finish cooling temperature of first cooling was equal to the Bs temperature of each steel.
- microstructure The microstructure, yield strength, tensile strength, yield ratio, Charpy impact transition temperature, and ammonia stress corrosion cracking (SCC) test were performed on the steel sheet prepared as described above, and the results are shown in Table 3.
- a sample of the microstructure was taken from the 1/4t portion of the steel sheet, mirror-polished, corroded using a Nital corrosion solution, and observed using an optical microscopy, and thereafter, a phase ratio was obtained through an image analysis.
- a sample was taken from a 1/4t portion of the steel sheet, mirror-polished, corroded using a LePera corrosion solution, and observed using an optical microscope, and thereafter, a phase ratio of the MA phase was obtained through an image analysis.
- a sample of No. JIS4 was taken from a 1/4t portion of the steel sheet in a direction perpendicular to a rolling direction and subjected to a tensile test at room temperature to measure yield strength, tensile strength and A yield ratio.
- low-temperature impact toughness a sample was taken from a 1/4t portion of the steel sheet in a direction perpendicular to the rolling direction to manufacture a V-notch test sample and Charpy impact test was performed three times at each temperature at temperatures from -20 to -100°C at an internal of 20°C to derive a regression equation of each temperature average value, and low-temperature impact toughness was obtained at a temperature of 100J as a transition temperature.
- ammonia stress corrosion cracking (SCC) test was carried out using the test solution under the test conditions described in Table 4 by making proof ring test samples. 80% of actual yield stress was applied, and samples which were not broken for 720 hours were evaluated as pass and samples which were broken before 720 hours were evaluated as fail.
- SCC ammonia stress corrosion cracking
- inventive examples satisfying the compositions and manufacturing conditions proposed in the present disclosure are steel having excellent ammonia stress corrosion cracking (SCC) resistance, as well as having high strength and high toughness, and having a yield ratio of 0.8 or less, low yield ratio characteristics.
- microstructure of the inventive example A-1 was observed with a microscope and the results showed that the microstructure was a mixed structure including, in area %, 60% of more of acicular ferrite and the balance including at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA) as illustrated in 1-(2) of FIG. 1 .
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PCT/KR2016/015156 WO2017111526A1 (ko) | 2015-12-23 | 2016-12-23 | 응력부식균열 저항성 및 저온인성이 우수한 저항복비 고강도 강재 |
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KR101999018B1 (ko) * | 2017-12-24 | 2019-07-10 | 주식회사 포스코 | 저온인성이 우수한 후강판 및 그 제조방법 |
KR102164097B1 (ko) * | 2018-10-26 | 2020-10-12 | 주식회사 포스코 | 황화물 응력부식 균열 저항성이 우수한 고강도 강재의 제조방법 |
KR102307946B1 (ko) * | 2019-12-09 | 2021-09-30 | 주식회사 포스코 | 내해수성이 우수한 구조용 강판 및 이의 제조방법 |
KR102326109B1 (ko) * | 2019-12-16 | 2021-11-16 | 주식회사 포스코 | 황화물 응력부식 균열 저항성이 우수한 강재 및 이의 제조방법 |
KR102400036B1 (ko) * | 2020-04-13 | 2022-05-19 | 주식회사 포스코 | 저온인성이 우수한 저항복비 강판 및 그 제조방법 |
CN112342458B (zh) * | 2020-09-01 | 2022-01-11 | 南京钢铁股份有限公司 | 一种低屈强比抗应力腐蚀开裂高强钢及制备方法 |
CN113832399B (zh) * | 2021-09-23 | 2022-10-11 | 马鞍山钢铁股份有限公司 | 一种经济型抗硫化氢腐蚀管线钢及其生产方法 |
CN113913695B (zh) * | 2021-10-13 | 2022-10-18 | 鞍钢股份有限公司 | 耐腐蚀抗疲劳水下油气采输用管线钢及其生产方法 |
CN114005602A (zh) * | 2021-11-02 | 2022-02-01 | 兰州理工大学 | 一种低碳高强度低电阻率电缆线芯材、制备方法及应用 |
JP7364137B1 (ja) * | 2022-04-20 | 2023-10-18 | Jfeスチール株式会社 | 鋼板およびその製造方法 |
WO2023203702A1 (ja) * | 2022-04-20 | 2023-10-26 | Jfeスチール株式会社 | 鋼板およびその製造方法 |
JP7401838B1 (ja) | 2023-05-18 | 2023-12-20 | 日本製鉄株式会社 | 液体アンモニア中応力腐食割れ特性の評価方法 |
JP7401839B1 (ja) | 2023-05-18 | 2023-12-20 | 日本製鉄株式会社 | 液体アンモニア中応力腐食割れ特性の評価方法 |
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JPH0417613A (ja) | 1990-05-12 | 1992-01-22 | Nippon Steel Corp | 耐応力腐食割れ特性に優れた高張力鋼の製造方法 |
JPH07188742A (ja) * | 1993-12-28 | 1995-07-25 | Kawasaki Steel Corp | 低温用鋼の製造方法 |
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JPH0987802A (ja) * | 1995-09-21 | 1997-03-31 | Kobe Steel Ltd | 耐めっき割れ性に優れた高張力鋼板およびその製造方法 |
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KR101304824B1 (ko) * | 2009-12-28 | 2013-09-05 | 주식회사 포스코 | 라인 파이프용 api 강판 및 그 제조방법 |
KR101181247B1 (ko) * | 2010-04-20 | 2012-09-10 | 한국기계연구원 | 변형능과 저온인성이 우수한 기가급 초고강도 강판 및 이의 제조방법 |
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JP5842359B2 (ja) * | 2010-10-28 | 2016-01-13 | Jfeスチール株式会社 | 非調質低降伏比高張力鋼板およびその製造方法 |
KR20130110643A (ko) * | 2012-03-29 | 2013-10-10 | 현대제철 주식회사 | 강판 및 그 제조 방법 |
KR101465088B1 (ko) * | 2012-08-17 | 2014-11-26 | 포항공과대학교 산학협력단 | 저온 인성이 우수한 저탄소 고강도 강판 및 그 제조방법 |
KR101482359B1 (ko) * | 2012-12-27 | 2015-01-13 | 주식회사 포스코 | 극저온 인성이 우수하고 저항복비 특성을 갖는 고강도 강판 및 그의 제조방법 |
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- 2016-12-23 WO PCT/KR2016/015156 patent/WO2017111526A1/ko active Application Filing
- 2016-12-23 US US16/063,886 patent/US20180371588A1/en not_active Abandoned
- 2016-12-23 JP JP2018532057A patent/JP6691217B2/ja active Active
- 2016-12-23 CN CN201680075892.9A patent/CN108431274B/zh active Active
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CN108431274A (zh) | 2018-08-21 |
JP6691217B2 (ja) | 2020-04-28 |
US20180371588A1 (en) | 2018-12-27 |
KR20170075933A (ko) | 2017-07-04 |
JP2019504200A (ja) | 2019-02-14 |
CA3009137C (en) | 2021-04-13 |
KR101767778B1 (ko) | 2017-08-14 |
EP3395987B1 (de) | 2020-04-29 |
CA3009137A1 (en) | 2017-06-29 |
EP3395987A4 (de) | 2018-11-07 |
WO2017111526A1 (ko) | 2017-06-29 |
CN108431274B (zh) | 2021-12-07 |
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